No, this episode must be a final episode. It is the last episode of season 1 (The 13th should also be the first of season 2 but let's move on...) You respected that though with the groups of episodes, be consistent I don't know? Seriously...

I see where you’re coming from, but it also feels a bit odd to have a "season finale" in the middle of a TMDB-designated season and more than one in such a "season".

I wonder what the consensus among mods/admin staff is with this "season finale" episode type as there are no restrictions to setting any or all episodes in a season with that label/type.

After thinking about @Vivienb’s comment, I've changed my mind and agree that episode 12 should have the season finale tag. Can it be it changed to that please, otherwise, we need a reason as to why it shouldn’t be?

My thoughts:

• I've looked at Oshi no Ko, and for episode 11 (the end of season 1), another mod adjusted that to said type and locked it at that. There are probably more examples like this.
• I do stand by my comment about it being a bit odd to have more than one season finale episode in a season, but then, that's only as a result of continuing the season "one" designation on TMDB as per that controversial rule, so this issue can't be helped.